Magnetic recording media are having to meet increasingly stringent requirements with regard to recording, playback and aging resistance. The binder is becoming increasingly important for meeting these requirements.
For example, to improve the magnetic properties, a higher packing density of the magnetic material is desirable in the layer, resulting in a reduction in the binder content of the layer. Attempts are also being made to achieve an improved signal/noise ratio by the use of increasingly finely divided magnetic materials having a pronounced acicular shape. These materials are furthermore very often surface-modified to reduce aging phenomena. As a result of such measures, both the division of the pigments in the dispersing process and the achievement of good dispersion stability are made considerably more difficult. In general, however, the magnetic layers must be very flexible and have high elasticity and good tensile strength. Furthermore, to avoid drops in output level, a reduction in the coefficiency of friction, an increase in abrasion-resistance and better wear properties are increasingly being required. In addition, the mechanical stability of the magnetic layer must also be ensured, particularly at high temperature and high atmospheric humidity.
It is known that magnetic layers which are subjected to strong mechanical stresses contain polyurethane elastomers which prove to be advantageous as binders. Polyesterurethanes as described in DE-B 1 106 959, DE-B 2 753 694, EP-A 0 069 955 or U.S. Pat. No. 2,899,411 have particularly proven useful.
However, these binders are not suitable in conjunction with the requirements and measures described above. In many cases, pigment wetting and pigment dispersion are adversely affected, so that any material sintered during the milling process is not sufficiently broken down or agglomeration of the pigment particles is not sufficiently prevented, leading to poor magnetic properties and hence to poor tape properties in respect of electroacoustics and video data. Relatively small amounts of low molecular weight dispersants are therefore added to facilitate the dispersing process. Although these dispersants have good deagglomeration properties, they stabilize the dispersion poorly. Higher molecular weight dispersing resins are therefore often used as additional components. DE-A 30 26 357 and DE 31 37 293 describe, for example, the addition of polyester resins which have SO.sub.3 M groups.
However, these methods for improving the dispersing process have disadvantages. For example, low molecular weight dispersants may readily be exuded in unfavorable climatic conditions, such as high temperature and/or high atmospheric humidity. This results in deposits on all parts in contact with the tape, particularly on the head, in recording and playback apparatuses, causing drops in output level. Furthermore, the friction (adhesion) increases greatly and may result in stopping of the tape, i.e. blocking. On the other hand, when dispersing resins are used, there may be compatibility problems in the dispersion. These substances are frequently not film formers and thus also cause blocking. Moreover, the mechanical properties of these dispersing resins are often not matched with the properties of the polyurethane used as the main binder. A deterioration in the mechanical properties also always means an increase in abrasion.
Since the amount required depends on the number of square meters (m.sup.2) of pigment surface area, the use of low molecular weight dispersants leads to a total amount of dispersant which is unacceptable for the properties of the tape. Since these dispersants contain polar groups, the hydrophilicity of the layer increases greatly and hence, especially in a humid climate, also effects such as the following:
Swelling of the layer PA1 Exudation of the dispersants and lubricants PA1 Changes in the mechanical properties due to changes in the plasticizer effects. PA1 a good dispersing effect PA1 rapid dispersion PA1 good stabilization of the dispersion PA1 low solvent requirement PA1 good rheology of the dispersion, i.e. low flow limit PA1 and not too high a viscosity PA1 good leveling on casting PA1 high pigment content of the layer PA1 good orientability of the magnetic needles PA1 good mechanical properties of the magnetic layer even at high temperatures. PA1 B1) one or more diols of 2 to 18 carbon atoms and PA1 B2) a diol having one or more sulfonate groups, PA1 m is from 0 to 50, PA1 n+m.gtoreq.1 and PA1 X is --SO.sub.2 M, where M is H, Li, Na, K or ammonium and M.sup.1 and PA1 M.sup.2 may be identical or different and are each H, Li, Na, K or alkyl.
Consequently, there is a greater tendency of the tape to block and soiling of the head is more likely.
To improve the dispersing properties of the polyurethane binder itself, it was proposed at an early stage to incorporate polar groups into the binder. These polar groups can in principle be introduced via any component which is used in the preparation of the polyurethane. Polyesters having polar groups are most frequently used (cf. inter alia DE-A 28 33 845). Incorporation of diols which additionally carry polar groups is described in, for example, JP-A 57 092 421, German Laid-Open Application DOS 3,814,536 and EP-A 193 084. The subsequent incorporation of the polar groups by nucleophilic substitution (S.sub.N) reaction at the terminal OH groups of the polyurethanes is disclosed in JP-A 57 092 422. The polyurethanes described to date and carrying polar groups have improved dispersing behavior but the improvement is still insufficient for many requirements.
A further disadvantage of all polyurethanes described is that the required elasticity is frequently accompanied by insufficient hardness and a tendency to exhibit surface tack. Combining corresponding polyurethanes with other binders is therefore part of the prior art. Proposed binder combinations are, for example, mixtures of polyurethanes with phenoxy resins, with vinylidene chloride/acrylonitrile copolymers, with vinyl chloride/acrylate copolymers or with polycarbonates or polyesters. DE-A 32 39 160 may be mentioned by way of example. Although these binder combinations lead to an improvement in the mechanical properties of the magnetic layer, the dispersing behavior of such a combination is adversely affected. The particular properties of the magnetic materials thus have an unsatisfactory effect. This is evident from a lower orientation ratio, lower residual induction and hence lower sensitivity at short and long wavelengths and a more unsatisfactory output level of the resulting recording media.
A possible method for increasing the hardness of the polyurethanes is to increase the concentration of urethane and urea groups. However, such measures very rapidly lead to products which are insoluble in conventional solvents, such as methyl ethyl ketone, toluene or tetrahydrofuran (EP-A 01 43 337). According to DE-A 31 37 293, nonmagnetic particles are admixed for increasing the hardness.
However, the measures described are not sufficient for simultaneously meeting the increased requirements for the binder system. Moreover, a combination of the polyurethanes with other binders is often essential for achieving individual effects.
However, the disadvantages of using high molecular weight binders are the high solvent requirement, the relatively long dispersing times and/or the necessary 2-phase dispersing. Furthermore, the particular properties of the magnetic materials are not satisfactorily displayed in these binder systems. This is shown by the low orientation ratio, low residual induction and hence low sensitivity at short and long wavelengths and an unsatisfactory output level of the resulting recording media.
An important improvement in the dispersing behavior is achieved by means of low molecular weight OH-containing polyurethanes as described in EP 0 099 533. However, these measures too are insufficient for dispersing increasing finely divided pigments and for meeting the growing requirements for the magnetic recording media with regard to mechanical and magnetic properties.
It is an object of the present invention to provide a binder system for extremely finely divided ferromagnetic pigments having a BET surface area &gt;40 m.sup.2 g.sup.-1 for novel high density recording systems, such as S-VHS in the video sector, DAT in the audio sector or novel computer tapes (MTC) in the data sector, which binder system meets the abovementioned requirements of